In typical wood frame construction, a builder will attach floor joists directly to a joist header (also known as a rim board, header joist, rim joist, rim band or band) with mechanical fasteners such as nails and wood screws. The builder then cuts pieces of fibrous insulation, e.g., glass fiber insulation, or rigid insulation to fit between each joist to insulate the joist header and prevent air infiltration. In some cases, the builder will caulk along the cracks between the rigid insulation, joists and joist header. Occasionally, the builder will spray foam insulation, e.g., a sprayable polyurethane, into these cracks and other spaces where there is no rigid insulation. With the exception of the latter construction practice, the 7″ to 13″ of wall height at the ends of joists tend to provide poor thermal protection, e.g., due to improperly installed glass fiber insulation that sags, air leakage through the fibers of fibrous insulation, or air leakage around cracks between pieces of rigid insulation and the floor joists where they meet the joist header.